It is known that certain irregular movements may occur to spring systems which are subjected to impulsive vibrational forces such as valve springs for intake and exhaust valves of internal combustion engines and suspension springs for vehicles due to the resonant movements of the springs. Japanese patent publication No. 47-22363 discloses a coil spring having an uneven winding pitch as an attempt to obtain a nonlinear spring property and raise the resonant point of the spring system.
In compression coil springs, end portions are typically ground into planar shapes so that their longitudinal center lines may stay straight even when they are substantially compressed. In particular, in high precision applications, it is common to use coil springs having closed and ground ends.
However, the closed ends do not contribute to the effective number of turns of the coil winding, but rather increase the overall height of the compression coil spring in fully compressed state, and increase the overall weight of the coil spring. This problem is particularly pronounced when the coil spring has an uneven winding pitch.